The invention applies to the acquisition of images, particularly in the infrared range and, more precisely to observation cameras designed to output a video signal to a display monitor to form images complying with the standard used in the monitor.
The invention provides a multi-standard observation camera whose output video signal can be adapted to several television standards.
More specifically, but not exclusively, the invention applies to thermal cameras and can be used in panoramic or sectorial surveillance systems.
To convert the light flux received from a scene observed into a video signal, thermal cameras conventionally include a detector, placed in a cryostat, the detector consisting principally of a large number of sensor elements; the sensor elements are sensitive to a specified infrared spectrum band and capable of outputting a quantity of charges proportional to the illumination applied to them. The charges output are transmitted to a processing and multiplexing circuit; charge transfers in such a circuit are triggered by pulses generated by a clock sequencer adjusted to form video signals complying with the standard used in the display monitor.
The sensor elements are placed horizontally relative to the final image formed from the video signals output by the camera and displayed on the monitor.
In general, the sensor elements are arranged in several rows parallel to the same horizontal direction and the detector is used in conjunction with an optomechanical system which projects and vertically scans an image of the scene projected onto the detector. The detector then analyzes the scene using the same line or frame pattern as the image displayed on the monitor, the geometry of the lines being fixed by the display standard.
The layout of the sensor elements in several rows is designed to meet several objectives:
to provide an adequate number of sensors within a given detector length, for example 6.33 mm; PA1 to comply with Shannon's sampling theory concerning signal sampling frequency; this implies the detection zones must overlap; PA1 to increase the detector sensitivity by summing, with a delay and using a method known to the prior art, signals originating from different detection zones but corresponding to the same image point in the scene observed successively. PA1 to comply with the sampling theory, the rows are horizontally offset by one half of the size of a sensor along this same direction; the output signals are then resynchronized in the processing circuits, in synchronization with the vertical scan and with partial overlapping of luminance signals; PA1 to improve sensitivity, each row is repeated several times with no horizontal offset; the processing circuit sums signals from corresponding sensors, lying on the same vertical line due to the vertical scan, and corresponding to the same image point, synchronously with the scan, using a mode known to the prior art and referred to as "Time Delay and Integration" or by its abbreviation TDI. PA1 a detector of a given length, sensitive to a given spectrum band and consisting of sensors arranged in rows over the length of the detector, all in the same horizontal direction, to form a given type of structure, PA1 an optomechanical projection and scan system to project an image of the scene observed, the width of the image being virtually equal to the length of the detector, onto a focal plane coinciding with the detector plane and to sweep this image vertically over the detector, PA1 a cryostat containing the detector and, connected to the detector, a read circuit which uses the charges the sensors output, proportional to the illumination they receive, after transfer and multiplexing under the control of a clock sequencer synchronized with the vertical scan, to output a video signal to a display monitor complying with a given television standard, this standard being defined by a line frequency and, in the case of digital TV, by a sampling frequency characteristic of the images displayed:
Conventionally, two types of row layout are used independently:
The problem is to design an image acquisition system which simultaneously satisfies the sampling and sensitivity requirements mentioned to produce a high-quality image which can be adapted to several television standards.
However, the first two requirements (sampling and sensitivity) require the use of a relatively large number of rows while the third requirement (adaptability to several television standards) means that the charge transfers are triggered independently for each row and must be different for each standard; this in turn implies generating a corresponding number of clock signals. These signals are generated by "proximity electronics" outside the cryostat and the number of connections outside the cryostat, proportional to the number of rows, will therefore be very high, leading to serious connection capacity and thermal linkage problems.
Consequently, up to the present time, the layout of sensors in offset or aligned sub-rows was compatible with only one standard and only one layout of rows, to the exclusion of any other: conventionally, compatibility with a standard is ensured by adjusting the pitch between rows to an integer sub-multiple of the pitch between the image or frame lines in the television standard in question; it then becomes possible to use only one type of sensor layout with a limited number of rows to limit the connection problems.
For example, SOFRADIR have developed a 228.times.4 sensor strip. This strip is used with a horizontal scan device since television standards are not quantitatively specified in this direction.